It is reasonably clear from past research that exposure to media violence on TV or in films both stimulates aggressive behavior in the short run and increases the risk for habitual aggressive behavior in the long run. The psychological processes involved in these effects seem to be observational learning of scripts for aggressive behavior and cognitions supporting behavior as well as habituation of emotional responses to violence in the long run and priming and arousal processes in the short run. There are theoretical reasons to expect violent video games to have an even stronger effect on aggression than violent TV programs. Some of the factors that have been shown to moderate the effects of film and TV violence are identification and active involvement with the characters portrayed, attention to the scenes, perception of them as realistic, vicarious reinforcement obtained from characters reinforcements, and actual reinforcements obtained when the scripts are imitated. Whereas TV viewing is a relatively passive activity, video game playing is highly active (e.g., the player shoots the gun). Most violent video games require the player to take on the identity of a violent game character (e.g., the player sees things from the game character's visual perspective). Most violent video games reward individuals for behaving aggressively (e.g., players get points for killing people). Compared to violent TV programs and films, the violence shown in violent video games is almost continuous. We propose to conduct a series of experiments and a longitudinal study to test the extent of the short term and long term effects of playing violent video games on the development of aggressive behavior and cognitions. Because the theory equally suggests that the experience of playing pro-social games should enhance pro-social behavior, we plan to investigate the effects of pro-social games as well. More than just demonstrating an effect, the experiments and longitudinal study are designed to investigate what personal and contextual factors moderate any effects, how the effects vary with age of the player, and what cognitive and emotional changes mediate any effects on behavior. The obtained results can be key to designing and testing interventions to reduce the short and long term impact of repeated exposure to violent video games and also to designing games that can enhance pro-social behavior and appropriate psychosocial adjustment.